The present invention relates to thin film coatings for steel components used in fuel injectors, and more particularly for metal carbon material thin film coatings for low alloy steel or tool grade steel fuel injector needles.
Many internal combustion engines, whether compression ignition or spark ignition engines, are provided with fuel injection systems to satisfy the need for precise and reliable fuel delivery into the combustion chamber of the engine. Such precision and reliability is necessary to address the goals of increasing fuel efficiency, maximizing power output, and controlling emissions or other undesirable by-products of combustion.
In direct injection diesel engine applications, a fuel injector is a precision device that must meter the quantity of fuel required for each cycle of the engine and must develop the high pressure necessary to inject the fuel into the combustion chamber at the correct instant of the engine operating cycle. Mary fuel systems presently used in direct injection diesel engines utilize a hydraulically actuated and/or electronically controlled fuel injector to pressurize the fuel charge to obtain the desired fuel spray pattern and fuel volume into the combustion chamber at the precise moment.
Additionally, the many modern hydraulically actuated and/or electronically controlled fuel injectors often operate in a much more harsh or severe environment, in terms of operating temperatures, pressures, speeds, etc. than conventional fuel injectors. These hydraulically actuated and/or electronically controlled fuel injectors often use very compact and high precision moveable components such as the fuel injector needles, valves, and plungers to achieve the prescribed delivery of fuel at the desire time and for the desired duration.
As a result of the compact nature of many fuel injector components together with the harsher environment, the stresses and forces present within an operating fuel injector are often concentrated on such smaller components. The decreased surface area of the smaller components in which to spread out the higher contact forces and stresses causes fuel injectors and fuel injector components to experience increased adhesive and abrasive wear at the contact surfaces, such as the nozzle tip and check interface. In addition, fuel injectors and fuel injector components require superior harness characteristics and lubricity characteristics to combat the higher contact stress.
The operability and reliability of a fuel injector is dependent, to some extent, on the fuel to be injected, and in particular on the lubricity, viscosity or other salient physical characteristics of the fuel to be injected. The use of low lubricity fuels, in particular, can cause several problems, and most notably fuel injector component wear (both abrasion and adhesion wear), which leads ultimately to the fuel injector tip failure or overall performance degradation of the fuel injector. Such form of wear is typically caused by lack of lubrication at the interface between two hard surfaces causing a welding or adhesion of the contacting parts, e.g. the exterior surface of the fuel injector needle and interior surface of the fuel injector sliding and impacting against one another without proper lubrication tend to show evidence of severe wear. These forms or patterns of wear will change the clearance between the exterior surface of the fuel injection needle and the cooperating interior surface of the injector body or guide surface and will make the surfaces rough so the sliding motion of the components will not be smooth, both of which will lead to an incorrect amount of fuel injected into the system. Eventually, continued wear can lead to failure of the fuel injector tip and/or needle valve. As indicated above, wear patterns of fuel injector components is particularly evident in fuel injection systems that utilize low lubricity fuels.
Various related art techniques have considered the use of titanium nitride (TiN) coatings on fuel injector plungers and other components to reduce wear of the coated parts. A problem encountered with TiN coatings is that a TiN coating is usually applied at extremely high temperatures (e.g. about 450 degrees C.) which may produce unwanted thermal stresses and related failures to the fuel injector components. It is also believed that TiN coatings on fuel injector needle tend to increase the wear of the needle mating component at the mating or seating location. In addition, TiN coatings tend to increase the overall cost of the fuel injector needle because the TiN coated fuel injector needle requires tool grade steel to withstand the high temperatures observed in the application of the TiN coating.
Alternatively, several related art techniques have considered the use of ceramic materials as the base material for the fuel injector needle instead of low alloy steels. Unfortunately, the use of a ceramic material for fuel injector needles is very costly and the resulting monolithic ceramic fuel injector needles have not typically demonstrated the necessary durability or reliability for commercial use in diesel engines or other heavy duty engine applications.
The present invention aids in overcoming one or more of the aforementioned problems associated with the fuel injector needle members and satisfactorily addresses the shortcomings of the related art solutions to such problems.
The invention may be characterized as a metal carbon material or diamond like carbon coated low alloy steel or tool grade steel component for a fuel injector, such as a fuel injector needle, having suitable hardness characteristics, improved boundary lubrication characteristics, and improved wear resistance characteristics. The fuel injector component, such as a fuel injector needle or other contacting surface, comprises a low alloy steel or tool grade steel substrate and a primary thin film coating (e.g. titanium containing diamond like carbon, chromium containing diamond like carbon, or tungsten containing diamond like carbon) deposited on the low alloy steel or tool grade steel substrate. The primary coating preferably has a thickness generally no greater than about 2.0 microns. The preferred coating may also include a thin bond layer (e.g. less than 1.0 micron thick chromium layer) deposited between said low alloy steel or tool grade steel substrate and the primary metal carbon material coating.
A further aspect of the present invention is the resulting hardness, boundary lubricity, and wear resistance characteristics of the coated steel substrate. For example, the hardness of the metal carbon material coating is preferably greater than 1000 Kg/mm2 as measured using a Hardness Knoop HDNS 50 gram load whereas the boundary lubricity and wear resistance characteristics of the coated substrate are generally improved over those of an non-coated steel substrate.